In a lithium ion secondary battery, a porous membrane, which is mainly constituted by polyolefin, is used as a separator. In the porous membrane, fine holes formed in the porous membrane are clogged in a case where an abnormal current occurs and a battery temperature rises, and the like. Accordingly, the porous membrane has a shut-down function of blocking a current flow. According to this, the porous membrane becomes effective from the viewpoint of avoiding battery thermal runaway.
Examples of a technology related to the separator include a technology described in Patent Document 1.
Patent Document 1 (Japanese Unexamined Patent Publication No. 2012-129115) discloses a separator for a nonaqueous electrolyte battery including a porous substrate which includes polyolefin having a characteristic viscosity of 5×105 Pa·s to 5×106 Pa·s and non-Newtonian flowability of 0.15 to 0.4, and in which the degree of crystallinity is 65% to 85%, and a heat-resistant porous layer that is provided on at least one surface of the porous substrate and includes a heat-resistant resin.
Patent Document 1 describes that the separator for the nonaqueous electrolyte battery has excellent mechanical strength and a shut-down function, and is excellent in short-circuit resistance at a high temperature.